As a readily available monomer, ethylene has widespread use in the plastics industry. Historically, the price of butadiene, which is a by-product in the production of ethylene in petroleum processing, was comparable to that of ethylene. Recently, the process of ethylene production has changed, resulting in a decrease in the output of, and a marked increase in the price of, butadiene. In contrast, the price of ethylene has decreased. Therefore, using ethylene as a raw material for rubber products, such as those used in pneumatic tires, is attractive. For example, replacing butadiene with 10 wt % of ethylene in production of polybutadiene used in tires could lead to large savings in raw material costs. However, since the polymerization of a conjugated diene (such as butadiene) and that of an α-olefin (such as ethylene) occur under different mechanisms, their copolymerization is difficult. Therefore, it is challenging to copolymerize ethylene and a conjugated diene by the same catalytic system.